Materials which, in use, come into contact with whole blood, plasma, or other biological systems should not artificially disrupt the biological status quo of the system. This desirable characteristic is sometimes generally referred to as the "biocompatibility" of the material.
It has been observed that certain hemodialysis membranes can, when in contact with blood, activate the human complement system. It is believed that this contributes to a phenomenon called hemodialysis leukopenia, which is a temporary sequestration of leukocytes in the pulmonary vascular system of a small number of patients undergoing hemodialysis.
Cellulosic membranes manufactured by the cuprammonium process (also referred to as regenerated cellulose) have been associated with higher incidences of complement activation than other membrane materials, such as polyacrylonitrile, polycarbonate, and cellulose acetate. See, for example, Chenoweth et al, "Anaphylatoxin Formation During Hemodialysis: Effects of Different Dialyzer Membranes", Kidney International, Vol. 24 (1983), pp. 770-774; Chenoweth, "Biocompatibility of Hemodialysis Membranes", ASAIO, April-June 1984, Vol 7, No. 2, pp. 44-49; Chenoweth, "Complement Activation During Hemodialysis: Clinical Observations. Proposed Mechanisms, and Theoretical Implications, " Artificial organs, 8(3), 1984, pp. 281-287; and Chenoweth et al., Compartmental Distribution of Complement Activation Products in Artificial Kidneys, "Kidney International Vol 30, pp. 74-80 (1986).
As is shown in FIG. 1A, the surface of regenerated cellulosic membranes includes appended hydroxyl groups (designated OH) and perhaps, as impurities, appended amino groups (designated NH.sub.2). These groups are nucleophilic in nature, meaning that they have the natural tendency to seek out atoms with a lowered electron density, such as reactive acyl carbon atoms.
By their chemical nature, it is believed that the nucleophilic groups vary in the degree to which they seek out acyl carbon atoms. The amino groups are believed to be more reactive than the hydroxyl groups.
The human complement system includes certain plasma proteins, which are identified in FIG. 1A as Protein C3; Proteins C5; and Protein Factors H&I. These proteins are normally inactive. However, with appropriate stimulus, the Proteins C3 and C5 participate in an enzymatic cascade known as complement activation. The Protein Factors H&I serve as inhibitors to control or regulate the complement activation process.
As shown in FIG. 1B, it is believed that, when the plasma proteins come into contact with the nucleophilic surface, such as a regenerated cellulose membrane, a portion of the Protein C3 having activated carbonyl groups (identified as Component C3b) covalently binds to the surface nucleophiles. Another portion of the Protein C3 (identified as Component C3a) is split off and diffuses into the plasma. As this is occurring, a portion of the Protein C5 (identified as component C5b) also adheres to the membrane surface, freeing component C5a into the plasma. These cleavage components C3a and C5a are called anaphylatoxins.
Complement activation on nucleophilic surfaces like the regenerated cellulosics mentioned above occurs because the control Protein Factors H&I do not associated very well with C3b bound to these types of surfaces. Therefore, Factors H&I cannot inhibit the cascade from proceeding.
As shown in FIG. 1C, the freed C5a anaphylatoxin activates granulocytes, causing them to aggregate or become hyperadherent to the pulmonary vasculature. It is believed that this sequestration of the C5a-activated granulocytes manifests itself as leukopenia during dialysis.
The C3a anaphylatoxin does not appear to interact with the biological constituents of the blood. However, its presence can be measured in the blood, and C3a serves as a marker for accurately quantifying the degree of complement activation taking place.
It is believed that the incidence of complement activation associated with polysulfone, polyacrylonitrile, and polycarbonate hemodialysis membranes is relatively low because these membrane materials do not have appended nucleophilic groups. Certain polyacrylonitrile membranes have also been observed to actually bind the C3a and C5a anaphylatoxins, which are cationic in nature, removing them from circulation to limit patient exposure to these bioactive polypeptides.
In cellulose acetate, the nucleophilic characteristics of the regenerated cellulose are modified by covalently reacting an acetate group with some of the nucleophiles (see FIG. 4B). This reduces the number of bio-reactive nucleophiles available to covalently bind the C3b complement protein.
Efforts continue to develop ways of making regenerated cellulose membranes as biocompatible as polysulfone, polyacrylonitrile, polycarbonate, and cellulose acetate membranes. This is in part due to the efficiencies and low costs now associated with the manufacture of regenerated cellulose membranes, compared to other dialysis membranes.
In published European Application No. 0 172 437, it is claimed that the incidence of complement activation in regenerated cellulose membranes can be reduced by bulk blending unmodified regenerated cellulose with regenerated cellulose that has been modified with diethylaminoethyl, sulfopropyl, carboxymethyl, or phosphonate alkyl. As described in this published Application, about 4 to 40 parts by weight of the unmodified cellulose is blended with 1 part by weight of the modified cellulose.
As in the case of cellulose acetate, the number of bio-reactive nucleophiles associated with the modified cellulose is reduced, as the substituents become covalently bound to some of the nucleophiles (see FIG. 4A). The bound substituents also carry a negative charge and can bind the cationic C3a and C5a anaphylatoxins, removing them from circulation. However, in the blending process described in this published Application, only the nucleophiles associated with the modified cellulose are effected. All of the nucleophiles associated with the unmodified cellulose, which constitutes the major constituent of the blend, retain their natural bio-reactive nature.
In addition to biocompatibility, there are other desirable characteristics for materials intended for use as dialysis membranes. One such characteristic involves the ability of the membrane to remove Beta-2-microglobulin and other poorly dialyzed molecules in the 1000 to 20000 molecular weight range. Because regenerated cellulose material does not have the capacity to reduce Beta-2-microglobulin levels, the accumulation of this material in the tissue of dialysis patients has become a focus of concern in the medical community. See, for example, Gejyo F., Homma N., Suzuki Y., Arakawa M.: "Serum levels of a B-2-microglobulin as a new form of amyloid protein in patients undergoing long-term hemodialysis." N Eng J Med 314 (1986) 585-6; Zingraff J., Beyne P., Bardin T., Touam M., Uzan M., Man N. K., Drueke T.: "Dialysis amyloidosis and plasma B-2 microglobulin." Abstracts EDTA-ERA 23 (1986) p. 162; Vandenbroucke J. M.: "Relationship between membrane characteristics and dialysis induced changes in B-2-microglobulin levels." Abstracts EDTA-ERA 23 (1986) p. 156 ; Hauglustaine D., Waer M., Michielsen P., Goebels J., Vanduputte M.: "Hemodialysis membranes, serum B-2 microglobulin, and dialysis amyloidosis (letter)" Lancet 24 May 1986 p. 1211; Granollaras C., Deschodt G., Branger A., Oules R., Shaldon S., Floege J., Koch K. M.: "B-2-microglobulin kinetics during hemodialysis and hemofiltration (abstract)" Blood Purification 4(4) 1986 p. 210; and Vincent C., Revillard J. P., Galland M., Traeger J.: "Serum B-2-microglobulin in hemodialyzed patients." Nephron 21 (1978) 260-268.
One of the principal objectives of this invention is to improve the biocompatibility of materials having nucleophilic groups, such as regenerated cellulose. More particularly, it is a principal objective of this invention to provide a process of efficiently treating a material having nucleophilic groups in a way that blocks the covalent bonding of complement proteins and the associated release of anaphylatoxins.
Another principal objective of this invention is to provide a material which will bind bioactive materials, such as the anaphylatoxins, to limit overall patient exposure to the bioreactive materials.
Another principal objective of this invention is to provide a material capable of removing Beta-2-microglobulin and other like material from a biological fluid.